1. Field of the Invention
The present invention relates to an optical transmission device performing a light transmission through an optical fiber.
2. Descriptions of the Related Arts
Optical transmission devices used between instruments close to each other such as audio visual instruments and FA instruments have mounted 660 nm-band red LEDs, and have been used at transmission speeds ranging from several Mbps to 100 Mbps. As such optical transmission devices, plastic fibers measuring 200 to 980 μm in diameter, which are suitable for a wavelength of 660 nm-band, have been used.
In recent years, as digital devices have become more advanced, a high speed transmission among them has been required. Thus, laser diodes which can operate at a speed higher than LEDs have been used for optical transmission devices. However, since surface emitting-type laser diodes oscillate a laser light of 850 to 1550 nm-band, transmission loss becomes large with usage of the plastic fiber having a core measuring 200 to 980 μm in diameter. Accordingly, to decrease the transmission loss, optical fibers having a core measuring 50 μm in diameter have come to be used.
FIG. 18 is a cross section view showing an outline of a conventional optical transmission device 10 taken along an optical fiber 14 (for example, see Japanese Patent Laid-Open Publication [KOKAI] 2002-344024). The optical transmission device 10 comprises a light emission element 2 mounted on a lead frame 1 and a peripheral IC 3 for driving a light emission element 2. A lead frame 1, a light emission element 2 and a peripheral IC 3 are electrically connected by a gold wire 4. A light emission element 2, a peripheral IC 3 and a gold wire 4 are sealed by resin 5. A lens 6 is formed in front of a light emission surface of a light emission element 2. With such structure described above, a lead frame 1, a light emission element 2, a peripheral IC 3, a gold wire 4, resin 5 and a lens 6 constitute a light emission unit 7.
A receptacle 8 has an insertion port 11 for inserting a light emission unit 7 and a sleeve 12 for inserting an end of a optical fiber 14 which transmits an optical signal. An optical fiber 14 is constituted by a core which forms its center, and a clad surrounding the core, and the core and the clad are not identified in the drawing. A light emission unit 7 is inserted through a insertion port 11 of a receptacle 8, and fixed thereto with adhesive 9. An optical connector 20 is composed of an optical fiber 14, a connecting connector 15 and a ferrule 16. An optical fiber 14 is fitted to a central portion of a ferrule 16, and a tip portion of a ferrule 16 is designed so as to be flush with an end face of an optical fiber 14. Note that the boundary between an optical connector 20 and a receptacle 8 is represented by the thick lines.
By inserting a ferrule 16 in a sleeve 12, the center axis of an optical fiber 14 is positioned in line with the center axis of a sleeve 12. Lights emitted from a light emission element 2 are collected by a lens 6, and enter an optical fiber 14 through a sleeve 12.
When an optical signal enters an optical fiber 14 through a lens 6, an efficiency of coupling between a light emission element 2 and an optical fiber 14 is determined by a positional precision of a lens 6 and an optical fiber 14. Specifically, the efficiency of coupling between a light emission element 2 and an optical fiber 14 becomes higher, as the center axis of a lens 6 agrees more fully with that of an optical fiber 14.
Nevertheless, the size of an inserting port 11 has been made somewhat larger than the measurements of a light emission unit 7. Therefore, a light emission unit 7 deviates from a specified position in a receptacle 8 forward or backward, upward or downward and left or right. Alternatively, a light emission unit 7 occasionally may tilt relative to a receptacle 8. In this case, when a light emission unit 7 is fixed to a receptacle 8 with adhesive 9, it has been difficult to make the center axes of a lens 6 and an optical fiber 14 agree with each other. In other words, in a conventional optical transmission device 10, it has been difficult to determine the relative positions of an optical fiber 14 and a light emission element 2 precisely.
In a case where the diameter of the optical fiber 14 is 200 μm or more, if the center axes of a lens 6 and an optical fiber 14 disagree somewhat with each other, this has an insignificant effect on the efficiency of optically coupling between an optical fiber 14 and a light emission element 2. However, in a case where a diameter of an optical fiber 14 is equal to about 50 μm, the disagreement of the center axes of a lens 6 and an optical fiber 14 with each other has a significant effect on the efficiency of optically coupling between an optical fiber 14 and a light emission element 2. In the future, when a diameter of an optical fiber 14 is set to be further smaller than the present diameter thereof, the disagreement of the center axes of a lens 6 and an optical fiber 14 will have a more significant effect on the efficiency of optically coupling between an optical fiber 14 and a light emission element 2.
The respective center axes of a lens 6 and an optical fiber 14 are not constant among a plurality of optical transmission devices 10. This implies that the efficiency of coupling between an optical fiber 14 and a light emission element 2 varies for each optical transmission device 10.